Active matrix liquid crystal displays (AMLCDs) typically generate colored images by providing pixels which consist of three separate dots, each of which has a color filter transmitting one primary color. These dots usually cover an area of one third of a full pixel and are generally referred to as sub-pixels of the full pixel. As a result of limitations in process and design of the dots, the aperture for transmitted light is reduced in AMLCD displays resulting either in low brightness or high power in the backlight.
An alternative method for generating colored images is to have just one dot per pixel space, and sequentially flash the backlight within one image buildup period with the three color primaries, where the image build up period is the time in which all image information is output by the display such that a viewer is able to observe a full color image. This creates what is known as a color sequential display. The liquid crystal pixel can then sequentially control the amount of each primary color transmitted. Because the sequential flashing occurs quickly, the eye will integrate the light of one image buildup period such as to perceive a full color image.
A similar display technology is known as spectrum sequential display, and this technology only requires that the backlight is flashed twice per image buildup period. Color is then generated by each backlight flash in the form of two primaries (for example blue and yellow in the first sub-frame and cyan and red in the second sub-frame). Each pixel is divided into two dots and each dot has a color filter which transmits one primary from each flash of the backlight (for example blue and cyan for the first dot and yellow and red for the second dot). This approach thus provides a compromise between the time available for each flash of the backlight and the size of each pixel dot.
The two approaches above each rely upon flashing of the backlight, and the desired color for each pixel is built up as a sequence of color outputs. These two approaches can both be described as a “sequential drive scheme”.
One advantage of a sequential drive scheme is that the resolution can be increased compared to a standard display, because there need only be one or two sub-pixels per pixel.
This increase in resolution is of general interest for LCD displays, but is of particular interest for autostereoscopic display devices, in which a display panel has an array of display pixels for producing a display, and a plurality of imaging means, such as lenticular elements or semi-transparent barriers, arranged over the display panel and through which the display pixels are viewed. Taking a display having lenticulars as an example for explaining the working principle of such view directing means, the lenticular elements are typically provided as a sheet of lenticular elements (lenticulars), each of which comprises an elongate lens element that may have a desired lens shape such as elliptical or semi-cylindrical. The lenticular elements extend in the column direction of the display panel (or slanted with respect to the column direction), with each lenticular element overlying a respective group of two or more adjacent columns of display pixels or sub-pixels. In an arrangement in which, for example, each lenticule is associated with two columns of display pixels (no slant angle), the display pixels in each column provide a vertical slice of a respective two dimensional sub-image, i.e. multiple views are directed into multiple directions. The lenticular sheet directs these two slices and corresponding slices from the display pixel columns associated with the other lenticules, to the left and right eye of a user positioned in front of the sheet, so that the user observes a single stereoscopic image. The sheet of lenticular elements (and each lenticular element) thus provides a light output directing function such that light output intended for the left and right eye is directed into two different views or view directions.
In other arrangements, each lenticule is associated with a group of more than two adjacent display pixels in the row direction. Corresponding columns of display pixels in each group are arranged appropriately to provide a vertical slice from a respective two dimensional sub-image. As a user's head is moved from left to right across a display a series of successive, different, stereoscopic views are perceived creating, for example, a look-around impression. A detailed explanation of the working principle including slanting of the lens to achieve certain improvements is provided hereinafter and for example in U.S. Pat. No. 6,069,650.
The above described device provides an effective three dimensional display. However, it will be appreciated that, in order to provide stereoscopic views, there is a necessary sacrifice in the resolution of the device, as different sets of display pixels are associated with different views. This sacrifice in resolution is unacceptable for certain applications, such as the display of small text characters for viewing from short distances. For this reason, it has been proposed to provide a display device that is switchable between a two-dimensional mode and a three-dimensional (stereoscopic) mode. However, this fails to address the problem of the loss of resolution in the 3D mode.
It will be seen that the use of a sequential drive scheme can restore some of the loss of resolution. In addition, by reducing the amount of filtering, the efficiency is improved. When color filtered pixels are used, the efficiency is reduced by roughly 67%.
Although sequential drive schemes can improve resolution and efficiency, a problem is the occurrence of color break-up, also known as the “rainbow effect”. This is the effect that the color visibility at different moments becomes visible by high motion of the displayed image and/or by high motion of the viewer (eyes). In most cases this artefact is perceived as very disturbing.